1. Field of the Invention
The present invention relates to a piezoelectric thin film component and the manufacturing method thereof. The present invention also relates to an inkjet-recording head using the piezoelectric component, and an inkjet printer using this [head]. The present invention more particularly relates to a new improvement of a piezoelectric thin film component where residual strain is minimized.
2. Description of the Related Art
An actuator using a piezoelectric thin film component, which converts electric energy into mechanical energy or vice versa, is used for a pressure sensor, a temperature sensor, an inkjet type recording head, and others. In the inkjet type recording head, a piezoelectric thin film component is used as an actuator to be the drive source of ink ejection.
This piezoelectric thin film component generally comprises a piezoelectric thin film comprised of polycrystalline substances, and a top electrode and bottom electrode which are disposed sandwiching the piezoelectric thin film. The composition of the piezoelectric thin film is generally a binary system in which the main component is lead zirconate titanate (hereafter xe2x80x9cPZTxe2x80x9d), or a ternary system where a third component is added to the binary system.
The piezoelectric thin films with such composition are generated, for example, by a sputtering method, sol-gel method, MOD process (Metal Organic Decomposition process), laser ablation method and CVD method. For example, a ferro-electric substance using the binary system PZT is noted in xe2x80x9cApplied Physics Letters, 1991, Vol. 58, No. 11, pp. 1161xcx9c1163xe2x80x9d. Also, piezoelectric materials using the binary system PZT is disclosed in Japanese Patent Laid-Open No. 6-40035, and in the xe2x80x9cJournal of the American Ceramic Society, 1973, Vol. 56, No. 2, pp. 91xcx9c96xe2x80x9d.
When a piezoelectric thin film component is applied to an inkjet type recording head, for example, a piezoelectric thin film (PZT film) with a 0.4 xcexcmxcx9c20 xcexcm film thickness is appropriate. The piezoelectric thin film needs a high piezoelectric charge constant, therefore it is normally necessary to perform heat treatment at a 700xc2x0 C. or higher temperature to grow crystal grains of the piezoelectric thin film.
When a piezoelectric thin film (PZT film) with a 0.5 xcexcm or higher film thickness is formed, performing heat treatment to obtain a high piezoelectric charge constant causes cracks inside the film, which is a problem.
A method disclosed in xe2x80x9cPhilips J. Res. 47 (1993) pp. 263xcx9c285xe2x80x9d is creating a sol or gel composition, baking at high temperature to crystallize the piezoelectric thin film, and repeating this process to increase the film thickness of the piezoelectric thin film.
The piezoelectric thin film created by this method, which has a multi-layered interface, cannot present good piezoelectric characteristics, and processability is poor. Repeating the heat treatment also leads to deterioration of piezoelectric characteristics, such as crystals losing orientation.
A piezoelectric thin film is normally formed on a bottom electrode, which is formed on a substrate, and heat treatment performed to form the piezoelectric thin film causes curvature and strain on the substrate, which is a problem. Also good adhesion is required between the bottom electrode and the piezoelectric thin film.
So the present inventor and others considered various ways to increase the piezoelectric charge constant of a piezoelectric thin film, and discovered that it is effective if the crystals of the piezoelectric thin film have a predetermined crystal orientation and columnar structure, and also have a crystal structure with a grain size of 0.1 xcexcm to 0.5 xcexcm (Japanese Patent Application No. 9-288757).
However, the present inventor and others further examined and discovered the following problem. When an electric field is applied to a virgin state piezoelectric thin film component, residual strain and polarization strain remain in the piezoelectric thin film component, even after the electric field is removed, and good piezoelectric strain characteristics (displacement characteristics) cannot be obtained. In other words, if an electric field is applied to a piezoelectric thin film and this is polarized, the domain (crystal grains) of the piezoelectric materials creating the piezoelectric thin film moves such that the polarization axis matches with the direction of the electric field. Then cavities are generated in the grain boundaries of the grains, which seems to be causing residual strain.
With the foregoing in view, it is an object of the present invention to provide a mechanism that excels in piezoelectric strain characteristics, eliminating the influence of residual strain in piezoelectric thin films.
After studying piezoelectric thin films that can achieve such an objective in various ways, the present inventor discovered a piezoelectric thin film which has the following characteristics.
No or few foreign substances exist or the abundance of foreign substances is low at grain boundaries, which are boundaries between crystal grains of the piezoelectric thin film, even after performing polarization processing (poling) on the piezoelectric thin film component. The width of the grain boundary is 5 nm or less. The crystal grain boundary is a discontinuous layer, which does not continue the orientation of adjacent crystal grains. Here, xe2x80x9cforeign substancexe2x80x9d shall mean the non-crystallized substance separated from crystal grains of the piezoelectric thin film.
According to the discovery of the present inventor, such a structure can be favorably implemented by applying the above-mentioned MOD process to manufacturing a piezoelectric thin film using a sol-gel method. Foreign substances are compounds which are formed by the constituent elements of PZT, but the composition is different from PZT. That is, foreign substances are, for example, non-crystallized substances separated from crystal grains of the piezoelectric thin film.
An example is lead oxide (PbO). The abundance of foreign substances after applying an electric field to the piezoelectric thin film component can be controlled such that residual strain is within a target range. For example, 2.5xc3x9710xe2x88x924 or less is preferred, or a range where the piezoelectric effect can be improved, mentioned later, is preferable. A piezoelectric thin film where foreign substances exist becomes the cause of residual strain after polarization processing, which is clear by the later mentioned x-ray diffraction analysis. The residual strain refers to the strain when the electric field strength is 0 kv/cm.
According to the present invention, the residual strain is small or almost nonexistent since the shift of the domain of the piezoelectric thin film caused by applying an electric field at polarization processing or at the driving of the piezoelectric thin film component is small. This means that the hysteresis characteristic of the piezoelectric thin film component with respect to an applied electric field is low. As a result, a piezoelectric thin film component which has good strain-voltage (electric field) characteristics and a large displacement can be provided.
For the piezoelectric effect, that is, displacement-voltage characteristics of the piezoelectric thin film component of the present invention, the piezoelectric constant d31 to be an index is 180 pC/N or more, and it is possible to obtain a 1.2 times or higher piezoelectric constant compared with a conventional type.
Displacement of a piezoelectric film is generated by the shift of the relative position of metal atoms and oxygen atoms when voltage is applied to the piezoelectric thin film component. As a result of the shift of domains, cavities are generated at the grain boundaries between domains (between adjacent crystal grains). And the existence of these cavities decreases the withstand voltage of the piezoelectric thin film.
The present inventor has confirmed that decreasing foreign substances decreases residual strain. Also in the present invention, decreasing the width of the crystal grain boundary of the piezoelectric thin film leads to control of the crystal grain boundaries themselves, which are the source of cavities which cause residual strain.
It is preferable that crystal grains of the piezoelectric thin film be columnar with respect to the top and bottom electrodes, and that the crystal plane orientation be a (001) plane orientation of the tetragonal system or a (111) plane orientation of the rhombohedral system. A (100) plane orientation is also acceptable.
Crystallization of a piezoelectric thin film by a sol-gel method or MOD process begins from the lower electrode side, so an improvement is required to control the crystal orientation of the piezoelectric thin film elements. For this, forming seed crystals to form crystals of the piezoelectric thin film on the bottom electrode is possible.
Seed crystals (crystal source) are comprised of titanium, and are formed on platinum crystals which comprise the bottom electrode, or on the crystal grain boundaries. The above piezoelectric thin film has a crystal plane orientation described above, and has a columnar crystal structure with a 0.1 xcexcn to 0.5 xcexcm grain size.
More preferably, a crystal of piezoelectric film grown using a crystal source formed on the crystal grain boundary of the bottom electrode as the nucleus is formed on a plurality of crystal grains of the bottom electrode. This improves adhesion between the piezoelectric thin film and the bottom electrode.
Another piezoelectric thin film component of the present invention is characterized in that the grain size of the above-mentioned bottom electrode is set to a preferable grain size for the above-mentioned piezoelectric materials to display piezoelectric characteristics, and the grain size of the above-mentioned crystals of the piezoelectric materials are grown with the above-mentioned crystal source as the nucleus, so that the grain size becomes almost the same value as or higher than the grain size of the above-mentioned bottom electrode.
An inkjet type recording head of the present invention is characterized in that the above-mentioned piezoelectric thin film component is comprised as an actuator. In one embodiment, the inkjet type recording head comprises a substrate where ink chambers are created, a diaphragm which seals one face of the ink chambers and has a piezoelectric thin film component in strain vibration mode fixed on the surface, and a nozzle plate which seals the other face of the above-mentioned ink chambers and has nozzle holes for ejecting ink, and the above-mentioned piezoelectric thin film component comprises the piezoelectric thin film component described above.
If island-like seed crystals are formed between platinum crystal grains comprising the bottom electrode, then crystals grow in a columnar structure using seed crystals as the nucleus and a crystal structure with a predetermined plane orientation is created. However, to obtain a (111) orientation, this island-like titanium is not used.
If the crystals of the bottom electrode are set to the preferable grain size for the above piezoelectric materials to display piezoelectric characteristics and the crystals of the above-mentioned piezoelectric materials are grown using the above crystal sources as the nucleus, then the crystal grain size of the above-mentioned piezoelectric materials can be a value almost equal to or higher than the grain size of the bottom electrode. In other words, a grain size which exceeds the grain size of the bottom electrode can be implemented because in this structure a crystal of the piezoelectric thin film can be formed on a plurality of crystals of the bottom electrode. If the crystal sources are formed on the grain boundaries which are hardly influenced by the orientation of the crystals of the bottom electrode, then the crystals of the piezoelectric thin film can be grown using the crystal sources as the nucleus, and as a result the targeted orientation of the crystals of the piezoelectric thin film can be implemented.
The present invention is also characterized in that in a piezoelectric thin film component comprising a piezoelectric thin film which strains when an electric field is applied, the residual strain of the above-mentioned piezoelectric thin film is 2.5xc3x9710xe2x88x924 or less. The piezoelectric constant is preferably 150 pC/N or more. The present invention is also characterized in that [the present invention] is an actuator using these piezoelectric thin film components as an oscillation source.